Mine drainage from abandoned sites is an international problem. In Pennsylvania, abandoned mine drainage is the largest non-point source of stream impairment. Over 4,600 miles (7,400 km) of Pennsylvania streams have been degraded by mine drainage. In many cases entire watersheds have been so severely impacted by mine drainage that essentially no or little aquatic life remains.
Passive systems are one available technology for treating polluted mine drainage. These systems typically use no electricity, require limited maintenance, and utilize environmentally-friendly materials such as limestone aggregate and spent mushroom compost to encourage a variety of natural processes to occur in a series of constructed ponds, beds, ditches, and wetlands. The goal is to provide economical, long-term, effective treatment while minimizing daily operation and maintenance needs and therefore costs. Passive components are typically selected based upon the often variable quality and flow rate of the mine drainage to be treated, preferred chemical and/or biological processes, and available construction space. While typically used to provide treatment of mine water, passive systems may also be used to treat other sources of polluted waters.
Many passive components consist of constructed beds or ponds filled with limestone to neutralize acidity, raise pH, and/or remove metals. One of the many effective components available to designers of passive treatment systems is the Horizontal Flow Limestone Bed (HFLB). An HFLB is an open, unburied, bed of limestone aggregate, which is commonly installed as the final component in a passive treatment system. The HFLB serves two major purposes. First, the HFLB provides an alkalinity boost to the final effluent, which adds buffering capacity to the stream which in many cases is needed to lessen the impact of other acidic sources downstream. Second, the HFLB effectively removes dissolved manganese from the water stream.
Historically, removal of dissolved manganese from mine drainage has been problematic and thought to require chemical treatment in order to raise the pH above 9. With the development of passive technology, dissolved manganese has been observed to form solids at a much lower pH (6 to 7). The exact mechanism is not completely understood, but biogeochemical factors such as low dissolved ferrous iron concentrations, high dissolved oxygen concentrations, available surface area, sufficient alkalinity, presence of certain microorganisms (bacterial and fungal), and autocatalytic processes appear to play a significant role. The availability of certain nutrients, dissolved organic carbon, and other factors may also be important, depending upon the role and types of microorganisms in the removal process.
The HFLB, as well as many other effective passive treatment technologies such as vertical flow ponds, accumulate metal precipitates, sediment, vegetative debris, and various other contaminants. Over time, the accumulation of these materials results in decreased treatment efficiency as the treatment media becomes plugged and permeability decreases.
Before our invention, prior art methods for restoring permeability to the treatment media of these passive systems included flushing, backflushing, stirring, and other techniques. While these methods can be effective for some passive components, for others the impact to the overall functionality has been minimal or short lived. In some cases, the treatment media had become so coated with metals or the void spaces had become so plugged that the treatment media was actually removed, discarded, and subsequently replaced even though the media itself still possessed significant treatment capabilities. Decreased functional life expectancy of the component increases long term operation and maintenance costs and can lead to a perception that passive treatment is too costly, ineffective, and/or unreliable.
Accordingly, the present invention satisfies a need for a method of media rehabilitation that not only restores efficacy and functionality of the water treatment component, but also facilitates reuse of viable treatment media and recovers metal-containing material accumulated in the media as a valuable resource. Media rehabilitation saves money by extending the life of the treatment media. In addition, sale of the recovered metals and/or products made from the recovered metals may offset the cost of rehabilitation or possibly even generate profit. Another desirable aspect of our invention is that the recovery system is readily portable (even to remote locations) with a quick set-up time. Additional objectives and advantages of the invention will become apparent from the following detailed description of some particularly preferred embodiments.